Power sources of the VSCF type typically used in aircraft or aerospace applications comprise a synchronous generator which is coupled to the output of a variable speed prime mover, a rectifier/filter which converts the variable frequency output of the generator into DC power on a DC link, an inverter which converts the DC power on the DC link into constant frequency AC power and an optional filter which is coupled between the inverter and a load bus by controllable contactors. A further power source, for example another VSCF system, may be provided and may be connectable to the load bus by a second set of controllable contactors.
There are numerous applications where it is necessary or desirable to couple the VSCF system and the further power source in parallel across a load connected to the load bus. In such applications, it is important that no significant real and reactive power be transferred between the VSCF system and the AC power source so that losses are minimized and the load is shared equally between the system and the source.
A prior VSCF paralleling control disclosed in Baker et al. U.S. Pat. No. 4,728,806 operates switches in the inverter to control the flow of power between the VSCF system and the further power source. More specifically, real and reactive current demodulators develop real and reactive current error signals wherein the first represents the difference between the real currents supplied by the power sources and the second represents the difference between the reactive currents supplied by the power sources. The phase displacement of the VSCF output relative to the output of the external AC power source is controlled in accordance with the real current error signal. The output voltage of the VSCF system, however, is determined only in accordance with the reactive current error signal. It is believed that the use of only reactive current error to control inverter output voltage results in a system which can, under certain circumstances, become unstable under load.
In addition to the foregoing, the Baker et al. control does not accomplish individual phase regulation and regulates DC link voltage only by controlling generator exciter current. One consequence of this is that the parallel combination of power sources become "soft" due to the reduction in generator excitation when DC link voltage rises under no load conditions. Such a reduction in excitation causes the generator of the VSCF system to, in effect, disappear from the parallel combination so that it cannot supply significant power to a subsequently applied load until after a significant period of time has elapsed. This problem is said to be obviated in the Baker et al. control by sensing total real current error, i.e. the difference in real current flowing in all of the inverter phases, and using same to control inverter output phase displacement. Such a solution, however, does not directly address the underlying reason for the problem and is thus not an optimal solution.